It is well known that many polymers can be produced as powders in fluid bed reactors where the fluidization of the polymeric solids is provided by a circulating mixture of gases including one or more monomers. For example, vapor phase polymerization is a common process, widely used for the production of polyolefins, such as polyethylene, polypropylene, and polyolefin copolymers. One particularly arrangement of a fluid bed polyolefin process is disclosed in U.S. Pat. No. 4,882,400. Other examples of fluid bed polyolefin technology are described in, for example, U.S. Pat. Nos. 3,709,853; 4,003,712; 4,011,382; 4,302,566; 4,543,399; 4,882,400; 5,352,749; 5,541,270; 7,122,607, and 7,300,987. These patents disclose gas-phase polymerization processes wherein the polymerization medium is either mechanically agitated or fluidized by the continuous flow of gaseous monomer and diluent.
The “traditional” gas-phase fluidized-bed reactors described in many of the patents listed above is a simple and cost-competitive device useful for the manufacture of polyolefins. However, many desired polyolefin products are difficult to produce in such gas-phase reactors due to the well-mixed CSTR nature of the reactors, including bimodal and multimodal products or products having broad molecular weight distributions and other advanced products. Such products typically require the use of specialized catalysts, such as dual site or bimodal catalysts or the use of multiple reactors in series.
One attempt to overcome the deficiencies in a traditional gas-phase reactor is described in U.S. Pat. No. 5,698,642 disclosing a multi-zone circulating reactor (MZCR) in which there is a up-flowing riser section operating in a dilute-phase fast fluidization regime and a down-flowing dense-phase moving-bed section. The gas compositions in those two sections are set differently to achieve the product differentiation.
WO 2006/022736 discloses a reactor system composing a plurality of MZCRs connected in fluid communication, and describes different types of operation for the different reactor zones.
With respect to the MZCR, the dense phase down-flowing moving bed may be agglomeration prone and may cause significant problem in reactor operation. Pre-polymerization is required for the MZCR, although it can not solve all the agglomeration-related reactor-operation problems. For example, see P. Cai, I. D. Burdett, “Polymerization Simulation Under Different Fluidization Regimes,” Circulating Fluidized Bed Technology VIII, ed. by K. Cen, p. 410-417, International Academic Publishers (2005). In addition, it is very difficult to control the temperature uniformity in the down-flowing moving bed, which in turn can result in a negative impact on product property control.
Efforts have been reported to improve the operation of MZCR, such as adding liquid to the multiple locations in the down-flowing moving bed (e.g., EP 1,720,913). However, the non-fluidized dense-phase nature of the down-flowing moving-bed section of the MZCR does not allow a significant process improvement.
Another limitation of the MZCR is that the gas compositions in the two reactor sections can not be too far apart (e.g., any component absolutely undesired in the riser can not be fed into the downcorner), which limits the product capability. The production rate in the downcorner section is also limited because of the need to prevent particle melting by the reaction heat, which in turn limits the composition flexibility of bimodal or multimodal products.
Accordingly, there exists a need in the art for gas-phase reactors capable of producing a broad range of products without the need for specialized catalysts or multiple reactors.